At present, a type where a transmission liquid crystal display panel having a color filter is irradiated from a rear surface side by a backlight device so that a color image is displayed has been a mainstream type of a liquid crystal displaying device. In addition, although a CCFL (Cold Cathode Fluorescent Lamp) using a fluorescent tube has been conventionally and widely used as the backlight device, there is a limitation of use of mercury from the perspective of the environment. As a light source instead of the CCFL using the mercury, an LED (Light Emitting Diode) has been used. (See, for example, Patent Document 1.)
The liquid crystal panel backlight device can be classified into two types, namely an edge type or a direct type, depending on arrangement of the light source. In the edge type, a light guide plate is provided right under a rear surface of the liquid crystal panel and a light source is provided at a side surface part of the light guide plate. The edge type is used for a relatively small liquid crystal panel such as a mobile phone or a display of a notebook-type personal computer. In addition, in the direct type, the light source is provided right under the rear surface of the liquid crystal panel and is used for a large size liquid crystal panel because the direct type has a better utilization rate of light and less weight than the edge type.
In the direct type backlight device where the light emitting diode is used as the light source, there are two kinds of lighting ways. One is a way where a white color light emitting diode is used as the light source. Another is a way where a light emitting diode irradiating three primary colors, namely a red light, a green light, and a blue light, is used and a white light is obtained based on a mixture of the red light, the green light, and the blue light. There are two kinds of arrangement of the light emitting diodes in the direct type backlight device. One is, as shown in FIG. 1, where the light emitting diodes are uniformly arranged at the rear surface of the liquid crystal panel. Another is, as shown in FIG. 2, where the light emitting diodes are arranged in a line state, like a conventional CCFL (fluorescent tube). The arrangement shown in FIG. 1 is used for large size liquid crystal television screens or liquid crystal monitors. The arrangement shown in FIG. 2 is used for middle size liquid crystal television screens or liquid crystal monitors.
However, in the backlight device using the light emitting diodes arranged as discussed above, as well as the backlight device using the conventional CCFL, the light emitting diodes are always lighted at the time when the liquid crystal displaying device is being used. Accordingly, lower consumption of the electric power is further required. Because of this, as discussed in Patent Document 2, a structure has been suggested where the backlight is segmented into plural sub-units and brightness of the light emitting diodes are adjusted for every sub-unit so that the lower consumption of the electric power is achieved.    [Patent Document 1] Japanese Patent Application Laid-Open Publication No. 7-191311    [Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2004-191490